1. Field of the Invention
The present invention relates to a charged-particle beam system, such as an electron beam lithography system or a scanning electron microscope.
2. Description of Related Art
FIG. 1 schematically shows one example of a part of an electron beam lithography system. In this system, an image 3 of a beam source 2 is formed by a demagnifying lens 1. An image 5 of the image 3 is formed on the surface of a material 6 by an objective lens 4. Under this condition, if a deflector 7 is operated to deflect an electron beam 9, the image 5 can be moved into a desired position. Of course, the magnification (absolute value) of the demagnifying lens 1 is smaller than 1 because the electron beam (i.e., the image 5) of microscopic cross section should be focused onto the material 6. Where a high deflection speed should be obtained, an electrostatic deflector is used as the deflector 7.
Where a second deflector 8 is mounted in addition to the first-mentioned deflector 7 such that a multi-stage deflection system is constituted as shown in FIG. 1, the image 5 can be moved into a desired position while correcting deflection aberrations (e.g., deflection comatic aberration and deflection chromatic aberration). At this time, a deflection field produced by the deflectors 7 and 8 and having a strength and a sense is so selected that deflection aberrations caused by the deflectors 7 and 8 cancel each other out. If the deflector 8 is located close to the image 3, and if the electrode length is small, the deflection sensitivity of the deflector 7 that is the ratio of the distance traveled by the image 5 on the surface of the material 6 to the deflection voltage is higher, while the deflection sensitivity of the deflector 8 is lower. In this case, roughly speaking, the deflection distance and the sense of deflection on the surface of the material 6 are dominated by the deflector 7. Meanwhile, the deflector 8 corrects aberrations due to the deflector 7.
The deflection sensitivity of the deflector 8 is low. This means that as the deflector 8 is located closer to an object point (in this case, the image 3), virtual movement of the object point (image 3) produced by deflection decreases.
If there are two interlocking deflectors, such as the deflectors 8 and 7 as shown in FIG. 1, there are two degrees of freedom. This means that it is possible to determine the position of the image 5 and, at the same time, correct one kind of deflection aberration.
T. Hosokawa in Optik, Vol. 56, No. 1, pp. 21-30 (1980), teaches that if three interlocking deflectors are used, there are three degrees of freedom and that it is possible to determine the position of the image 5 and to correct two kinds of aberrations. Furthermore, it is set forth in JP59-083336 that even when there are two interlocking deflectors, one kind of aberration can be corrected and, furthermore, a second kind of aberration can be reduced (though it cannot be completely corrected) by appropriately selecting the intensity distributions in the lens field and in the deflection field.
Incidentally, in the prior art charged-particle beam system, when a charged-particle beam is deflected over a surface of a target, aberrations have been produced. Aberrations caused by deflection include curvature of field aberration, astigmatism, distortion aberration, comatic aberration, and chromatic aberration. Of these aberrations, curvature of field aberration and astigmatism produced as deviations of the focus can be corrected dynamically by correctors. Furthermore, deflection distortion aberrations produced as positional deviations can be corrected by superimposing a correcting signal onto the deflection signal. These corrections for aberrations have been already generally made, for example, in electron beam lithography systems.
Therefore, if the comatic and chromatic aberrations are also corrected, the lithography accuracy will be improved greatly. It is assumed here that the geometrical figure projected onto the material 6 is sufficiently small. Hence, errors caused by the size of the figure are not taken into consideration.
In order to correct deflection comatic aberration and deflection chromatic aberration at the same time while determining the position of the image 5, three deflections may be interlocked to provide three degrees of freedom. However, any guidances on designing such an optical system have not yet been given. That is, any guidances relevant to optimization of the deflection field and lens field have not been given. Depending on operational conditions, problems such as excessively high deflection voltage take place. In order to lower the deflection voltage without varying the magnitude of deflection, it is better to increase the deflector length or to reduce the inside diameter of the deflectors. Normally, it is difficult to increase the deflector length because of spatial restrictions. Furthermore, reducing the inside diameter of the deflectors will produce some problems including adhesion of contaminants on the inner wall of the deflectors and charging. Consequently, it is better to avoid this approach.
A charged-particle beam system in which the deflection voltage can be suppressed to low levels without increasing the deflector length or reducing the inside diameter of the deflectors is disclosed in JP2007-188937 (US2007/0158563) filed by the present applicant.
FIG. 2 illustrates the configuration of the charged-particle beam system. Those components of FIG. 2 which are identical with their counterparts of FIG. 1 are indicated by the same reference numerals as in FIG. 1. The system shown in FIG. 2 has an aberration-correcting deflector 8 located in a stage preceding the demagnifying lens 1. The deflection angle of the electron beam 9 is enlarged by making use of the focusing action of the lens.
However, even with this charged-particle beam system, the deflection voltages of all the deflectors may not be suppressed to low levels in cases where deflection comatic aberration and deflection chromatic aberration are simultaneously corrected. Where one kind of deflection aberration is corrected, positioning and correction of aberrations caused by the positioning can be assigned to two stages of deflectors, respectively. Where two kinds of deflection aberrations are corrected at the same time, three stages of deflectors are used. Therefore, in some cases, the effects of positioning or aberration correction made by some deflector are canceled by the effects of positioning or aberration correction made by another deflector.
In this case, it is necessary to increase the extent of the deflection performed by each deflector by an amount corresponding to the canceled amount of deflection for positioning or the amount of correction to deflection aberration (i.e., the angle through which the orbit of the electron beam 9 is swung rearwardly) in order to obtain a required magnitude of deflection. That is, the deflection voltage needs to be increased. Furthermore, the orbit is swung rearwardly through a large angle by strong deflection. This means that the orbit of the electron beam 9 more greatly deviates over a larger distance from the center axis of the lens. Consequently, there is another problem that other deflection aberrations, including field of curvature aberration, astigmatism, and distortion aberration, increase. Accordingly, even where this technique is used, it is necessary to more specifically determine the guidance on optimization of the design of the optical system that corrects deflection comatic aberration and deflection chromatic aberration at the same time.
It is set forth in the above-cited JP59-083336 that with the described optical system, one kind of aberration can be corrected and, furthermore, another kind of aberration can be reduced to a submicron level (where the deflection field measures 10 mm×10 mm) by using only two deflectors rather than three deflectors and operating them in an interlocking manner. However, it is impossible to completely correct the second kind of aberration. In the past, it has been recognized that there is no problem if beam blur due to deflection is reduced to a submicron level. In today's electron beam lithography, it is required that beam blur be reduced to nanometer levels or less and that, ideally, the beam blur be completely removed because of required high lithography accuracy.